I'm wondering what the logic behind the rejection was, because a project that I'm working on is considering something like this. Was it a technical decision (as in, too complicated to implement on the backend)? A UX decision (as in, it makes the UX too complicated)?

This other question suggests it was mostly to do with the implications on a user's reputation. Is that the case?

Atwood: … programmers love hierarchy, to a degree that they don't
even understand how different they are than the public in this regard.
Like they love putting everything in this little bucket, that goes in
this little bucket, which is this sub-bucket of this and this, and
normal people hate that. And threading is totally a manifestation of
that and it drives me crazy that a lot of programmers can't see that
they're like immediately like: "Oh, threading is good. I love
threading. What are you talking about?" You know? They can't see it at
all.

Spolsky: Right, right.

Atwood: It's like myopia.

Spolsky: Yeh. I mean it's really a function of the size of the
group, and one thing I've learned through years and years of usability
testing is that anything that smacks of a hierarchy or a tree is not
going to be understandable to the average, non-technical user.

Atwood: Yeh.

Spolsky: You just have to learn that: if it's a tree, or a
hierarchy, like eighty per cent of the regular people are going to get
confused and not quite get it.